Make compute_api use Aggregate objects

This makes the compute_api's AggregateAPI use the new Aggregate
object for its work.

Related to blueprint compute-api-objects

Change-Id: I97419bfe6c605306bd64f9aa4fce9657509df260
Fix a gross duplication of context code in objects tests

Because of the way we dual-inherit classes to get the local
and remote testing of objects, laziness caused us to duplicate
the code to get a context object all over the place. This
patch cleans that up.

Change-Id: I9b65968781afbecd0db72073299c019f8cc10241
Merge "Make compute_api use Aggregate objects"
Merge "Fix a gross duplication of context code in objects tests"